The invention relates to the determination of the position of the intersection of a beam of energy with a substrate.
Electron beam lithography (EBL) uses an electron beam to write patterns, e.g. mask patterns, in electron sensitive films on substrates. An electron beam can be focused to a diameter of less than 10 nm, allowing patterns of extremely fine dimensions to be written. As a result, EBL is used widely in industry for making photomasks and x-ray lithography masks, and in research for creating fine lines on a wide variety of substrates. The primary applications of EBL in the electronics industry include the production of photomasks with minimum feature sizes of 0.5 micrometers and larger, and the creation of "discretionary wiring" in integrated circuits.
X-ray mask making is an example of a task which requires extremely precise beam control. The need for precision arises in part because an x-ray mask (unlike most photomasks) is made at 1 to 1 scale, i.e., the pattern on the mask is of the same scale as is the pattern on the substrate. The features on photomasks (when used as reticles in photoreduction schemes) are scaled up, generally 5 to 10 times larger than features on the final substrate.
A major difficulty in the application of EBL to x-ray mask production (or, for that matter, the production of any 1-to 1-mask) lies in attaining precise placement of EBL-written features on the substrate. Precise feature placement is critical e.g., in achieving correct pattern "overlay". An integrated circuit generally requires the use of several different masking layers in successive stages of its fabrication. The patterns on these various layers must superimpose or "overlay" on top of one another to within a small faction (e.g., 0.1 to 0.3) of the minimum feature size. Consider, for example, the problem of fabricating an integrated circuit of 0.3 micrometers (300 nm) minimum feature size using x-ray lithography. If the minimum features on the mask are to be 300 nm wide, each of the features on the mask must be positioned in their assigned positions in X and Y to within a small fraction of 300 nm (e.g., to within 30 to 100 nm). If such precise positioning is not achieved, the patterns on the masking layers will not overlay. This precise positioning of all the parts of a pattern must be maintained over the entire area of the mask. Thus, a mask 3.times.3 cm in size might typically generate a positioning requirement of 30 nm out of 3 cm, or 1 part per million.
Precise positioning in X and Y of all parts of an EBL-generated pattern is difficult. The fundamental basis for this difficulty is best understood in the context of modern EBL metrology. The three most common approaches to writing in EBL are referred to as "raster-scan", "vector-scan", and "shaped-beam". In raster-scan methods (raster scanning involves side to side scanning similar to the scanning of a TV screen) an electron beam is scanned back and forth across the surface of the substrate. The beam is turned on and off at appropriate times to create the desired pattern in the electron sensitive layer. In order to minimize aberrations, distortion, and defocus of the electron beam, the length of the scan distance on the substrate surface is limited to about 200 micrometers. To create patterns over larger areas the stage holding the sample is moved, either continuously or in discrete steps. In this way, all areas of the sample can be brought into position to be written on by the beam. The stage position is generally monitored by a laser interferometer, which can measure stage position to less than 10 nm. If the stage is not precisely where it should be, a correction signal can be sent to the controls which scan the electron beam, and the beam can be appropriately deflected to compensate for stage position error.
A vector-scan EBL system operates in much the same way except that the beam is deflected only to positions at which pattern elements are to be written. The individual pattern elements are often written in raster fashion. During writing the stage is generally stationary, and writing takes place over only a limited field, typically square in shape, which contains the pattern elements. The maximum size of the field is usually about 10,000 times the beam diameter. Thus, for a beam 100 nm in diameter, the field might be 1.times.1 mm; for a beam 10 nm in diameter, the field might be 100.times.100 micrometers. Once the writing of the field is completed, the stage is moved to a new location, and another field written. As in raster-scan methods, stage position errors can be detected by the laser interferometer and corrected by appropriate deflection of the electron beam.
A shaped-beam EBL system also generally writes with the stage stationary. Instead of raster writing each pattern element, an entire shape (e.g., a square, a rectangle or other simple geometric figure) can be projected onto the substrate. For this reason shaped-beam EBL systems can usually perform a given task much more quickly than can vector-scan or raster-scan systems. The projected shape can also be scanned by means of appropriate deflection systems.
A fundamental problem common to the EBL systems described above arises from the way in which the position of the beam, relative to the substrate, is determined. In these systems the actual location of the electron beam on the sample, at any given instant, is not known unless the beam is caused to strike one or more fiducial marks on the sample. Fiducial marks are not always placed on samples prior to EBL. In those cases when they are, they are generally located outside the areas designated for writing. Fiducial marks are frequently used in EBL to position the beam at the start of a run, to adjust the orthogonality of scans in the X and Y directions, and to adjust the magnification scale in X and Y. Calibration relative to fiducial marks is typically performed under computer control.
Once the stage is moved so that the beam is no longer sampling the fiducial marks, all further positioning of the beam and the stage is done by "dead reckoning", with a laser interferometer monitoring only the stage position. The EBL systems discussed above do not directly detect the electron beam position once the stage is moved away from the fiducial marks. Thus, until the beam can be returned to the fiducial marks for recalibration, it is assumed that the beam does not undergo any spurious deflection and that beam deflection calibration is stable.
The use of dead reckoning to determine the position of the beam suffers from a number of intrinsic flaws. These flaws can lead to pattern placement errors on x-ray masks and other substrates. Some of the flaws arise from undesired stage movement, e.g., from pitch, roll, or yaw, which may accompany movement of the stage in X and Y. Error can also be introduced when the sample surface is not precisely coplanar with the laser interferometer beams that impinge on the stage mirrors. Moreover, the X and Y axes of the stage are not necessarily parallel with the X and Y axes of beam deflection, and the substrate surface plane is not necessarily orthogonal to the electron beam axis at all field locations. Lastly, there is always some drift or shifting of the electron beam scan field due to changes in temperature, residual magnetism in the stage, eddy currents, or electrostatic charging of the sample or other surfaces in the system. It is probably impossible to anticipate and correct for all sources of drift. Depending on the magnitude of the unpredictable drift, the dead reckoning approach is, at some minimum feature size, incapable of ensuring adequate overlay.